CMS-SMP-14-015

CMS-SMP-14-015 ; CERN-PH-EP-2016-013
Measurement of dijet azimuthal decorrelation in pp collisions at $ \sqrt{s} = $ 8 TeV
CMS Collaboration
13 February 2016
EPJC 76 (2016) 536
Abstract: A measurement of the decorrelation of azimuthal angles between the two jets with the largest transverse momenta is presented for seven regions of leading jet transverse momentum up to 2.2 TeV. The analysis is based on the proton-proton collision data collected with the CMS experiment at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 19.7 fb$^{-1}$. The dijet azimuthal decorrelation is caused by the radiation of additional jets and probes the dynamics of multijet production. The results are compared to fixed-order predictions of perturbative quantum chromodynamics (QCD), and to simulations using Monte Carlo event generators that include parton showers, hadronization, and multiparton interactions. Event generators with only two outgoing high transverse momentum partons fail to describe the measurement, even when supplemented with next-to-leading-order QCD corrections and parton showers. Much better agreement is achieved when at least three outgoing partons are complemented through either next-to-leading-order predictions or parton showers. This observation emphasizes the need to improve predictions for multijet production.
Links: e-print arXiv:1602.04384 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Distribution of $ { {E_{\mathrm {T}}^{\text{miss}}} / \sum { {E_{\mathrm {T}}} } } $ for data (points) in comparison with simulated jet production and other processes with large $ {E_{\mathrm {T}}^{\text{miss}}} $ (stacked) separately for the two regions $ {\Delta \phi _\text {dijet}} < \pi /2$ (a) and $\pi /2 < {\Delta \phi _\text {dijet}} < \pi $ (b). The main contribution of events with large $ {E_{\mathrm {T}}^{\text{miss}}} $ in the final state is caused by processes such as Z/W + jet(s) with ${\mathrm{Z} } \to \nu \bar{\nu} $ and $\mathrm{W} \to \ell \nu $.
png pdf	Figure 1-a: Distribution of $ { {E_{\mathrm {T}}^{\text{miss}}} / \sum { {E_{\mathrm {T}}} } } $ for data (points) in comparison with simulated jet production and other processes with large $ {E_{\mathrm {T}}^{\text{miss}}} $ (stacked) for $ {\Delta \phi _\text {dijet}} < \pi /2$. The main contribution of events with large $ {E_{\mathrm {T}}^{\text{miss}}} $ in the final state is caused by processes such as Z/W + jet(s) with ${\mathrm{Z} } \to \nu \bar{\nu} $ and $\mathrm{W} \to \ell \nu $.
png pdf	Figure 1-b: Distribution of $ { {E_{\mathrm {T}}^{\text{miss}}} / \sum { {E_{\mathrm {T}}} } } $ for data (points) in comparison with simulated jet production and other processes with large $ {E_{\mathrm {T}}^{\text{miss}}} $ (stacked) for $\pi /2 < {\Delta \phi _\text {dijet}} < \pi $. The main contribution of events with large $ {E_{\mathrm {T}}^{\text{miss}}} $ in the final state is caused by processes such as Z/W + jet(s) with ${\mathrm{Z} } \to \nu \bar{\nu} $ and $\mathrm{W} \to \ell \nu $.
png pdf	Figure 2: Normalized dijet cross section differential in $ {\Delta \phi _\text {dijet}} $ for seven $ { {p_{\mathrm {T}}} ^{\text {max}}} $ regions, scaled by multiplicative factors for presentation purposes. The error bars on the data points include statistical and systematic uncertainties. Overlaid on the data (points) are predictions from LO (dashed line; $\pi /2 \leq {\Delta \phi _\text {dijet}} < 2\pi /3$) and NLO (solid line; $2\pi /3 \leq {\Delta \phi _\text {dijet}} < \pi $) calculations using the CT10 NLO PDF set and excluding the bin at $ {\Delta \phi _\text {dijet}} =\pi $. The PDF, $ {\alpha _S} $, and scale uncertainties are added in quadrature to give the total theoretical uncertainty, which is indicated by the downwards-diagonally (LO) and upwards-diagonally (NLO) hatched regions around the theory lines.
png pdf	Figure 3: Ratios of the normalized dijet cross section differential in $ {\Delta \phi _\text {dijet}} $ to LO (triangles) and NLO (squares) pQCD predictions using the CT10 PDF set at next-to-leading evolution order for all $ { {p_{\mathrm {T}}} ^{\text {max}}} $ regions. The error bars on the data points represent the total experimental uncertainty, which is the quadratic sum of the statistical and systematic uncertainties. The uncertainties of the theoretical predictions are shown as inner band (PDF and $ {\alpha _S} $) and outer band (scales). The predictions using various other PDF sets relative to CT10 are indicated with different line styles.
png pdf	Figure 4: Normalized dijet cross section differential in $ {\Delta \phi _\text {dijet}} $ for seven $ { {p_{\mathrm {T}}} ^{\text {max}}} $ regions, scaled by multiplicative factors for presentation purposes. The error bars on the data points include statistical and systematic uncertainties. Overlaid on the data are predictions from the PYTHIA6, HERWIG++, PYTHIA8, MadGraph + PYTHIA6, and POWHEG + PYTHIA8 event generators.
png pdf	Figure 5: Ratios of PYTHIA6, HERWIG++, PYTHIA8, MadGraph + PYTHIA6, and POWHEG + PYTHIA8 predictions to the normalized dijet cross section differential in $ {\Delta \phi _\text {dijet}} $, for all $ { {p_{\mathrm {T}}} ^{\text {max}}} $ regions. The solid band indicates the total experimental uncertainty and the error bars on the MC points represent the statistical uncertainties of the simulated data.

Tables
png pdf	Table 1: The integrated luminosity for each trigger sample considered in this analysis.
png pdf	Table 2: The PDF sets used to compare the data with expectations, together with the corresponding maximum number of flavours $N_f$ and the default values of ${\alpha _S(M_{{\mathrm{ Z } } })} $.

Summary

A measurement is presented of the normalized dijet cross section differential in the azimuthal angular separation $ \Delta \phi_{\text{dijet}} $ of the two jets leading in $ p_{\mathrm{T}} $ for seven regions in the leading-jet transverse momentum $ p_{\mathrm{T}}^{\mathrm{max}}$. The data set of pp collisions at 8TeV centre-of-mass energy collected in 2012 by the CMS experiment and corresponding to an integrated luminosity of 19.7 fb$^{-1}$ is analysed.,The measured distributions in $ \Delta \phi_{\text{dijet}} $ are compared to calculations in perturbative QCD for 3-jet production with up to four outgoing partons that provide NLO predictions for the range of $2\pi/3 \leq \Delta \phi_{\text{dijet}} < \pi$ and LO predictions for $\pi/2 \leq \Delta \phi_{\text{dijet}} < 2\pi/3$. The NLO predictions describe the data down to values of $ \Delta \phi_{\text{dijet}} \approx 5\pi/6$, but deviate increasingly when approaching the 4-jet region, starting at $ \Delta \phi_{\text{dijet}} = 2\pi/3$, particularly at low $ p_{\mathrm{T}} ^{\text{max}} $. The pattern of increasing deviations towards smaller $ \Delta \phi_{\text{dijet}} $ and decreasing deviations towards larger $ p_{\mathrm{T}} ^{\text{max}} $ is repeated in the 4-jet LO region with $ \Delta \phi_{\text{dijet}} < 2\pi/3$, but with less significance because of the larger scale uncertainty.In a comparison of the normalized $ \Delta \phi_{\text{dijet}} $ distributions to the LOdijet event generators PYTHIA6, PYTHIA7, and HERWIG++, PYTHIA8 gives the best agreement. PYTHIA6 and HERWIG++ systematically overshoot the data, particularly for $ \Delta \phi_{\text{dijet}} \approx 5\pi/6$. A good overall description of the measurement is provided by the tree-level multijet event generator MADGRAPH in combination with PYTHIA6 for showering, hadronization, and multiparton interactions. The dijet NLO calculations from POWHEG matched to PYTHIA8 exhibit deviations similar to the LO dijet event generators, emphasizing thereby the need to improve the multijet predictions.Similar observations were reported previously by CMS[3] and ATLAS[4], but with less significance because of the smaller data sets. The extension to $ \Delta \phi_{\text{dijet}} $ values below $\pi/2$, the improved LO description in the 4-jet region $\pi/2 \leq \Delta \phi_{\text{dijet}} < 2\pi/3$, and the comparison to dijet NLO calculations matched to parton showers are new results of the present analysis.

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